A method using two cameras, called a stereo measurement method, is available to measure the three-dimensional coordinates of a feature point such as a corner of a product. In the stereo measurement method, as illustrated in a flow given below, three-dimensional measurement is performed using four steps to measure the three-dimensional coordinates of a feature point of a certain product:
Step 101 (calibration): The image distortion produced by the cameras is corrected in advance using an accurate calibration chart, and the focal positions of the two cameras and the relative positions of the respective sensor planes of the two cameras are determined.
Step 102 (image capture): Images are captured using the two cameras so that a feature point appear in the images.
Step 103 (find corresponding points): The three-dimensional coordinates of corresponding points corresponding to the feature point on the respective sensor planes are found using pattern matching or the like.
Step 104 (triangulation): The three-dimensional coordinates of the feature point are measured by performing three-dimensional measurement using a triangulation method from the two corresponding points determined in Step 103.
In the image capture processing of Step 102, it is necessary for the two cameras to have a common field of view. A narrow-angle lens having a narrow angle of view is used for high-accuracy measurement, which may be advantageous for the high measurement resolution. Unfortunately, the common field of view is small, resulting in a narrow range of three-dimensional measurement. Even with the use of a wide-angle lens, the close-up conditions provide a small common field of view and a narrow range of three-dimensional measurement for improved accuracy. Therefore, there is a trade-off between a high measurement accuracy and a wide measurement range.
To address the trade-off, PTL 1 discloses a method for providing a wide range of three-dimensional measurement by simulating movement of the human eyeball. In PTL 1, FIG. 3 of PTL 1 illustrates a camera in which a wedge prism is placed in front of a lens and the wedge prism is rotated to deflect the optical axis so that the capture position of each camera unit can be changed. Further, a combination of two wedge prisms allows independent control of the magnitude of a deflection angle (phi) and a deflection position (theta). The two camera units each including the above mechanism are used, and the fields of view are synchronously driven to scan the common field of view, thus ensuring a wide range of three-dimensional measurement. As disclosed in PTL 1, each camera unit captures an image using a narrow-angle lens so as to provide a sufficient accuracy or captures an image in a close-up mode, thus achieving both the high measurement accuracy and the wide measurement range.
Further, a method that allows independent control of the magnitude of a deflection angle and a deflection position to provide high-accuracy three-dimensional measurement even under bad capture conditions where vibration constantly occurs such as in a case where a three-dimensional measurement device is mounted in a robot arm or the like is also disclosed.
However, in three-dimensional measurement using the device disclosed in PTL 1, the wedge prisms are rotated to change the capture positions of the camera units. Thus, in addition to the normal two cameras, a mechanism including motors, gears, and prisms and a control circuit therefor may be required. Further, synchronous driving of a total of four prisms using two camera units may require four circuits each for controlling the driving of a motor capable of high-performance synchronization, leading to an increase in the size of a substrate for camera control. Furthermore, measurement may require the positioning time for driving the prisms, and an additional measurement time may be required accordingly.
Therefore, if the stereo measurement method disclosed in PTL 1 and a device using the method are mounted in a robot arm or the like, the mechanism or control circuits described above may be required in addition to two camera units, which may hinder downsizing of a robot arm having a stereo camera in terms of weight and size.